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Here at the Sloan Digital Sky Surveys our mission is to explore and map the Universe, from planets to the edges of the observable Universe. The way we do this is to collect light from specially selected objects we see in the night sky – but we can’t visit them in order to measure how far away they are. So how do we actually know how far away they are in order to make a map of the Universe?

Measuring the distance to objects in the Universe has always been one of the biggest challenges for astronomers. Until we know the distance to something we cannot really understand its physical properties, and the history of astronomy is full of examples where new techniques for measuring distances opened up entirely new areas of study. For example when the “spiral nebulae” were first discovered there was a long debate over if they were small clouds of gas in our own Galaxy, or external galaxies in their own right each made up of millions or billions of stars. Only by measuring their distances was this finally settled, and our understanding of the size of the Universe suddenly jumped many orders of magnitude.

A collection of “spiral nebulae”. But how can we tell that they are distant galaxies rather than nearby gas clouds? Credit: SDSS

There’s some really useful bits of physics we can use to help measure distances to the galaxies from their light. To do this we need to understand spectroscopy. Once SDSS had finished imaging more than a quarter of the sky with its camera, it became entirely focused on “spectroscopic” surveys. Our telescope in New Mexico collects the light from stars and galaxies and uses instruments called spectroscopes to split it up into its different colours (we actually have two different spectroscopes working right now – the APOGEE spectroscope and the BOSS spectroscope). These measurements split the light into a rainbow (or a spectrum), and we look for the precise colours of series of emission and/or absorption lines to tell us all sorts of things about the light source we’re looking at.

A hot bright light source (like a star) will have a “continuous spectrum” (with the peak colour depending on its temperature – hot things glow red, even hotter things glow white or blue hot). If the light from that passes through a cool cloud of gas before we measure it, that will create “absorption lines” where very specific colours (or “wavelengths” in proper scientific terms) are absorbed by atoms in the gas cloud. The exact pattern of colours/wavelengths which are absorbed tell you which atoms are in the gas cloud. If the gas cloud gets heated up enough we might instead see emission lines – at the same specific colours, where the atoms are now re-emitting these very specific colours/wavelengths. Each atom has a very distinctive pattern of lines it emits – for example hydrogen (the most abundant element in the Universe) has a very distinctive and bright emission/absorption line in the red part of the spectrum (at a wavelength of 656.3nm).

Emission spectrum of hydrogen in visible light (wikimedia commons)

Astronomers have been using this technique to work out the materials which make up the Sun and other stars for decades. It’s not always easy (it has been compared to trying to reconstruct a piano from the noise it makes falling down the stairs), but it works. When astronomers first used the technique to look at galaxies however they were very surprised by what they found. The patterns of lines seemed to be in completely the wrong places – for example the famous hydrogen lines weren’t even visible in some cases – they had moved right into the infra-red part of the spectrum.

In order to understand why this could happen we need to learn about another part of physics – the Doppler effect. First proposed in 1842, by a Physicist named Christian Doppler this is the observation that when a source emitting a wave is moving, the waves are shortened if the source is moving towards the observer, and lengthened if it is moving away. Most people are familiar with this effect when they have listened to ambulance sirens passing them on the street; the siren is higher in pitch when the ambulance is moving towards you and lower when it’s moving away (when sound waves are lengthened the pitch drops, and when they are shortened the pitch rises).

Wikimedia commons illustration of the Doppler effect.

Since light is a wave, the same effect happens when light is emitted from a moving source. When the waves of light are shortened the light becomes bluer, and when they are lengthened the light becomes redder.

An astronomer named Vesto Slipher, was the first person to try this out on galaxies, and he found that almost all galaxies he looked at showed enormous “redshifts”, implying that almost all the galaxies were moving away from the Earth at very high speeds.

Edwin Hubble is given the credit for explaining this observation by realising that we live in a Universe which is constantly expanding. In such a Universe any observer will observe almost all other galaxies moving away from them. Hubble published the first description of a relationship between how fast galaxies appear to be moving away from us (their “redshifts”) and their distances – this relationship is now called Hubble’s Law.

It is this relationship that we use to measure the distances to the galaxies from detailed observations of the light they emit, and astronomers are now used to describing the distances to galaxies as simply their “redshift”.

A map of the Universe from SDSS where the distance to galaxies is given in terms of their redshift. Credit: SDSS

A paper appearing in Nature today (Xue-Bing Wu et al. 2015, Nature, Feb 25) presents the most massive black hole discovered to date when the Universe, was less than a billion years old – just one-fifteenth of its current age.

A new method to select high-redshift quasars using SDSS observations combined with data from the WISE satellite has resulted in the discovery of new group of quasars at the far reaches of the universe, with redshifts greater than z = 5. One of these quasars, named SDSS J0100+2802, holds a super-massive black hole at a redshift of 6.3 when the Universe was only 900 million years old.

This black hole is estimated to have a mass 12 billion times that of our Sun. The existence of such a massive black hole at such an early stage in the Universe poses a deep mystery whose resolution will improve our understanding of how galaxies form.

The newly discovered quasar SDSS J0100+2802 is shown by the large red dot in the graph above. The graph shows that SDSS J0100+2802 has most massive black hole and the highest luminosity among all known distant quasars. The background photo, provided by Yunnan Observatory, shows the dome of the 2.4-meter telescope and the sky above it. (Image: Zhaoyu Li/Shanghai Observatory)

We are beginning a series of spotlights on APOGEE team members, with special emphasis on their interests in APOGEE science. This month, the spotlight is on Duy Nguyen, one of APOGEE’s postdocs. He graduated from the University of Toronto with a PhD in astronomy and astrophysics, and then held postdoc positions at the University of Florida, Stockholm University, and the University of Rochester before joining the APOGEE team.

Duy’s research is on the subject of binary stars. A binary star is actually two stars orbiting each other. The sizes of the binary star orbits are small enough that the two stars cannot usually be distinguished in images. This can confuse the interpretation of starlight; and in a survey like APOGEE where precise velocities of stars are so important, this can be a big hindrance. As a result, a number of different methods have been employed to try to tease out whether a star is a binary or not.

But this post isn’t just about binary stars — it’s about one scientist’s research into better understanding them! And in many ways, Duy sees APOGEE as the best available experiment for binary star studies. APOGEE takes multiple spectra of most stars in its sample over months and even years, and this time sampling enables orbital periods to be measured. APOGEE’s high spectral resolution means that tiny Doppler shifts in a star’s spectral lines can be measured precisely. And most importantly, such a large sample as APOGEE has observed (more than 150,000 stars to date) means that we may be able to get a better handle on the “binary fraction” of stars in the Milky Way — a problem that has been plaguing modern astronomers for decades.

Duy is primarily interested in the dynamic properties of binary stars. These dynamics are primarily observed by means of the Doppler shift. As the stars in the binary pair orbit one another, each star approaches and recedes from the Earth once per orbit. Every time they approach the Earth, their spectral lines move to a slightly smaller (or “bluer” to use astronomical lingo) wavelength. And every time the star recedes, the spectral lines move to a redder wavelength. These small changes can be detected with APOGEE, and the radial velocity variations of the stars can be determined based on how large is the wavelength shift.

Duy and his collaborators are amassing radial velocity information on the stars in the APOGEE sample, looking for candidates with substantial radial velocity shifts. When they find one, they fit the data points with an orbital model to determine what the most likely stellar masses are. Here is an example fit:

On the x-axis is the time in days, and on the y-axis is the velocity of the star relative to the Sun. This plot shows that the best fit to these data suggest that two stars, one that is at least 0.21 times the mass of the Sun and the other that is 1.6 times the mass of the Sun, are orbiting one another every 112.98 days at a distance greater than 0.065 A.U. It’s interesting to note that the less massive star in this binary is eight times smaller than its companion. Large mass discrepancies in binaries are typical, so that one star dominates the other in terms of brightness. This is one reason why binaries are so difficult to detect.

To date, about 12,000 possible stellar binaries from the APOGEE sample have been flagged based on radial velocity shifts, and 4,000 of these are of special interest because they have been visited seven or more times and exhibit significant radial velocity changes. Of these, 1,500 indicate stellar mass companions, such as the one figured above. While the 12,000 possible binaries were found automatically, the 1,500 sources with stellar mass companions have all had to be screened by hand — a process that Duy would like to fully automate.

Analyzing APOGEE’s huge repository of stellar spectra will enable the most comprehensive assessment of binary stars, including details about whether binary star characteristics are different across the Galaxy. And as an added bonus, APOGEE is sensitive enough to spot Jupiter-sized planets using these methods! How many planets are lurking in the APOGEE dataset?

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Special thanks to N. Troup, D. Chojnowski, and S. Majewski for assistance preparing this post.

Black holes are intriguing objects. A black hole is a phenomenon whose gravity is so strong that not even light, the fastest traveller in the Universe, can escape from its influence. Once thought mere oddities due to their extreme properties, today, black holes are found to be vital in the formation and lives of galaxies, including our own Milky Way.

But how do we know black holes exist if we can’t see them? Well, even if we can’t see a black hole directly we can observe their influence and indeed the energy and light emitted as gas, dust and stars fall into a black hole; that is, we can see black holes when they are actively “eating” material. When the supermassive black hole, which can be up to a billion times more massive than our Sun, at the center of a galaxy starts to eat new material the resulting process is so bright it can be seen out to ~200 billion lightyears away. Astronomers call the observational result of this process either an active galactic nuclei, or in the most extreme examples a “quasar”. So you might be surprised to find that an object that emits no light can cause the brightest known phenomenon in the Universe!

An artist’s rendition of a quasar created by Coleman Krawczyk (ICG Portsmouth). The image is drawn on a radial log scale with the central black hole 1 AU in size.

The light of quasars is not produced by the black hole itself, but instead it comes from the material, mostly gas, that is falling into the black hole. Different types of light are produced by this material at different distances outward from the black hole. Near the surface (or horizon) of the black hole (about the distance of the Earth’s orbit away for supermassive black holes in galaxies) this gas becomes extremely hot and produces X-rays. Stretching out from this to fill a region about the size of our Solar System, a disk of gas shaped like a frisbee is formed. The inside of this disk is closer to the black hole than the outside, so it rotates faster causing friction within the disk. This friction causes the gas to heat up and glow, producing light in the optical to ultraviolet part of the spectrum.

From the edge of the gas disk to a distance of about 3 light years (similar to the distance from the Sun to the next closest star), the temperature becomes low enough that particles of “interstellar dust”, made of carbon and silicon, form. These dust clouds form what is know as the “dusty torus,” a donut shaped ring round the gas disk. Some of the light coming from the gas disk is absorbed by the dust and re-emitted at longer wavelength infrared light. At very large distances from the black hole, some quasars have radio jets coming out along the poles. As the name suggests, this jets produce light at radio wavelengths cased by electrons being accelerated along a strong magnetic field. When these jets are present they can be up to ~300 thousand lightyears (~3 times the diameter of our entire galaxy!) in size.

Not only can a black hole produce light, it can create light at all wavelengths from the radio up to the X-ray, and across an area stretching from the size of the Earth’s orbit out to distances larger than the Milky Way. Therefore, growing black holes, and the regions around them are anything but “black.”

With discoveries from its earliest imaging campaigns, the SDSS extended the study of quasars back to the first billion years after the Big Bang, showing the rapid early growth of black holes and mapping the end stages of the epoch of reionization.

Stacked spectra of more than 46,000 quasars from the SDSS; each spectrum has been converted to a single horizontal line, and they are stacked one above the other with the closest quasars at the bottom and the most distant quasars at the top. Credit: X. Fan and the Sloan Digital Sky Survey.

With full quasar samples hundreds of times larger than those that existed before, the SDSS has given us the most accurate descriptions of the growth of black holes over cosmic history. SDSS spectra show that the properties of quasars have changed remarkably little from the early universe to the present day.

Growth in the number of known quasars in the largest homogeneous (solid) and heterogeneous (dashed) quasar catalogs as a function of time. The Sloan Digital Sky Survey catalogues started being produced in 2000. Fig. 1 from Richards et al. (2009).

SDSS studies have probed the dark matter environments of quasars through clustering measurements, revealed populations of quasars whose central engines are hidden by obscuring dust, captured changes in quasar spectra that show clouds moving in the gravitational grip of the central black hole, and allowed a comprehensive census of the much fainter accreting black holes (active galactic nuclei, or AGN) in present-day galaxies.

This, our first post for the IYL2015 is a collaboration between Coleman Krawcyzk (ICG Portsmouth); Nic Ross (ROE) with help from Karen Masters (ICG Portsmouth).

As astronomers, at the Sloan Digital Sky Survey everything we do is based on collecting light from cosmic objects. SDSS is therefore pleased that in 2015 we are celebrating the International Year of Light, and we especially would like to point out the Cosmic Light Theme, supported by the IAU.

As a small contribution to this celebration, every month in 2015 SDSS will have a special post on here talking about the different ways we use light. Our first post, which will appear before the end of January will be about how we use light to study black holes, something which seems like a contradiction, but has taught us a lot!

This post will be updated to collect all the links as the year progresses:

APOGEE surveyed 156,481 stars in its first three years. And of course APOGEE-2 is going to increase this sample size significantly. But to celebrate the successful end of APOGEE and the Data Releases 11 & 12 (also see here), we’d like to share with you a slice of the kind of data it collected.

Some background: The APOGEE/APOGEE-2 instrument collects near-infrared spectra of distant stars, and the survey is aimed at studying the history of the Milky Way Galaxy. How it does that is explained here. Along the way, it has taken spectra of each known spectral type: from hot O-type stars (with surface temperatures of about 30,000 degrees, or five times the surface of our own Sun) down to M-type stars (about 3,500 degrees, or roughly half the temperature of the Sun). Each of the spectral types (O, B, A, F, G, K, M) is defined based on how many and what kind of atomic or molecular species are seen in their spectrum. For instance, O-type stars have lots of singly-ionized atomic species visible in their spectra, whereas A-type stars have very strong hydrogen lines, and M-type stars have lots of neutral molecules, especially lines of TiO when you look in the visible portion of the spectrum.

These spectral types were defined using the visible portion of the spectrum. So when we look in the near-infrared, do they appear to be different? Here we go:

The O-type star spectrum looks pretty bland — the strongest lines due to ionized Helium in the near-infrared H-band are at 15721 and 16922 Angstroms (the line at 15271 Angstroms is due to interstellar molecules, and is therefore not from the star). The B-type star shows pretty significant absorption lines due to the Brackett series of atomic Hydrogen (those transitions beginning at the n=4 excited state), and those plus a whole bunch of smaller wiggles from other atoms can clearly be seen in the A- and G-type spectra as well. Below that and things look a lot more complicated. If you have experience with data like these, you might be tempted to think that the spectra of the G-, K-, and M-type stars are “noisy”, meaning that they weren’t observed for long enough and therefore weren’t detected well. But that’s not the case: every single spike visible in these spectra is due to an atomic or molecular transition that originates in the photosphere of the star!

All sorts of SDSS related stuff will be going on at this meeting, from dozens of talks and posters, to demos of SDSS online resources at the SDSS Booth in the Exhibit Hall and not to mention the final data release from SDSS-III. Our “Tweep of the Week” for this exciting week will be SDSS-IV Spokesperson, Jennifer Johnson.

Jennifer Johnson is an Asssociate Professor in the Astronomy Department of The Ohio State University. Her science interests are in stellar abundances, the origin of the elements, nucleocosmochronology and the formation of our own Galaxy and Local Group. She is the Science Team Chair of the APOGEE survey of SDSS-III, and the Spokesperson for SDSS-IV (as well as working on APOGEE-2).

Jennifer Johnson

The SDSS Spokesperson has two main roles. She is the main person in charge of making sure the SDSS collaboration is running smoothly and fairly. As part of this, the Spokesperson Chairs the SDSS Collaboration Council (which has a representative from each institutional member of SDSS). This group are the first point of approval for requests for Architect Status (ie. people who have contributed so much to SDSS development they can request to be on any publication) and External Collaborator requests (non-SDSS members working on specific projects), as well as for drafting our publication and other collaboration policies. They also organise the annual SDSS Collaboration Meetings (the next one to be held in Madrid, 20-23rd July 2015).

The SDSS Spokesperson is also responsible for representing SDSS to the press and the public. As such she is responsible for working with the SDSS Communications Director (Jordan Raddick) to draft the text of press releases and maintain the SDSS website, as well as with the SDSS Director of EPO (Karen Masters) on our collective public engagement and outreach efforts.

SDSS collaboration members gathered around the telescope at an unfortunately beautiful sunset.

The SDSS-III BOSS and SDSS-IV eBOSS are in the middle of a 4-day meeting to discuss the continuing great science coming out of BOSS, looking at the first data from eBOSS, and planning for the bright future of SDSS-IV. The location is Cloudcroft, New Mexico, which is only 17 miles from the Apache Point Observatory, home of The Sloan Foundation 2.5-meter Telescope, which has been the main telescope for SDSS for the past decade-and-a-half. This proximity allows for collaboration members to visit the telescope and meet the hardworking mountain staff who keep it all running smoothly.

Cloudcroft has been a central landing point for all of the years of the SDSS survey, and in recognition of this, honorary membership was granted to a certain permanent member of the staff at The Lodge Resort at Cloudcroft:

The SDSS’s map of the Universe shown as a pie diagram. Each dot is a galaxy; the colour indicates the local density (with red revealing the most dense places). This represents a slice through the Universe, with the Earth in the centre and galaxies further from the Earth plotted further from the centre (the distance is labelled here as redshift). The angle around the pie is marked by the sky co-ordinates (Right Ascension).

This pie diagram is one of the most famous images from the original phase of SDSS, which mapped the distances to 1 million galaxies out to a distance of about 2 billion light years (z=0.15, or 615 Mpc in comoving radius).

The map shows a slice through the Universe with the Earth at the centre, and each of the 1 million galaxies in the SDSS Main Galaxy Sample as a point. The points are colour coded by local density to hi-light the cosmic web (with red points in the highest densities).

The black parts of the pie are where SDSS did not map galaxies, either because our Milky Way is blocking the view from Earth, or because those parts of the Universe are not visible from our telescope in New Mexico.

Even while the Universe is expanding, all the matter in it clumps due to gravity and the structures we see in this map are the result of that. The details of the growth of these structures over time depends on both the expansion history of the Universe and the total amount of matter in it. So by accurately mapping the locations of galaxies in this map, scientists in SDSS have been able to measure both of these things making an important contribution to our knowledge of how the Universe works.

An optical image of the star SDSS J0018-0939, obtained by the Sloan Digital Sky Survey. This is a low-mass star with a mass about half that of the Sun; the distance to this star is about 1000 light years; its location in the sky is close to the constellation Cetus. (Credit: SDSS/NAOJ)

So obviously this mentions SDSS, but implies it’s a robotic telescope! Our team of observers, plate pluggers, and drillers, and the hundreds of other people who work hard to keep SDSS observing might object to this….

In March 2012, BOSS observed 103,000 spectra, each of which was routed through a fiber-optic cable that was plugged by hand. The industrious APO plugging crew is pictured here showing the deleterious effects of having placed more than 2,000 fibers/finger in a month. But don’t worry, they recovered have continued to plug every fibre optic by hand during the day at APO – they might even be doing it as you read this! (Image Credit: Dan Long, APO).

Amazing that our observing process is so smooth that to outsiders it appears to be like a robot! Stay tuned for a newly planned “The SDSS Telescope is not Robotic” article. :)

Cites SDSS as one of the reasons it was right that the invention of the CCD got the 2009 Nobel Prize in Physics because of the realms of discovery it opened up:

“The world could get along well without camera cell phones. What’s exciting about CCDs, whose inventors won the 2009 physics prize, is their use in the Hubble Space Telescope and the Sloan Digital Sky Survey.”

Authors use a sample of 50 non-barred and two armed spiral galaxies selected from SDSS and measure spiral arm pitch angles, finding most are only approximately log spiral, typically having decreasing pitch angle as radius increased. Link to paper.

Report on release of images from the Chandra archive, which us SDSS images (among many others) to make nice multi wavelength images, like the below one of NGC 4736.

NGC 4736 (also known as Messier 94) is a spiral galaxy that is unusual because it has two ring structures. This galaxy is classified as containing a “low ionization nuclear emission region,” or LINER, in its center, which produces radiation from specific elements such as oxygen and nitrogen. Chandra observations (gold) of NGC 4736, seen in this composite image with infrared data from Spitzer (red) and optical data from Hubble and the Sloan Digital Sky Survey (blue), suggest that the X-ray emission comes from a recent burst of star formation. Part of the evidence comes from the large number of point sources near the center of the galaxy, showing that strong star formation has occurred. In other galaxies, evidence points to supermassive black holes being responsible for LINER properties. Chandra’s result on NGC 4736 shows LINERs may represent more than one physical phenomenon. (X-ray: NASA/CXC/Universita di Bologna/S.Pellegrini et al, IR: NASA/JPL-Caltech; Optical: SDSS & NASA/STScI)

Oct 27th: “Nothing Can Escape Black Holes – this Lucky Star Did”, TechTimes
Study which revealed a star loosing a portion of its mass to a black hole used some SDSS data.

“I believe the large surveys shocked astronomers with how much data there is,” Hilbe says. “The Sloan Digital Sky Survey [one of the first automated and digitized comprehensive astronomical sky surveys] told them they needed statistics.”

Notice another mention of SDSS applying the process is automated, which we addressed above (thanks again to our wonderful observing team). Apparently this idea is fairly ubiquitous in the media….

Articles use an SDSS image to illustrate the LMT pointing at galaxy 5MUSES-229, one of the dusty galaxies in the study which was used to study the relative contributions of AGN and star formation in the heating of dust.

The LMT pointed at 5MUSES-229, a galaxy approximately one billion light years distant from the Milky Way. With the LMT, astronomers are able to observe the carbon monoxide emission from this galaxy. Credit: James Lowenthal, the background image showing the galaxy is from SDSS.

The OYRA (Macronix Prize) is given each year to one to two young, ethnic Chinese physicist/astronomer outside of Asia, in recognition of their outstanding achievements in physics/astronomy. The Award carries a cash prize of US $2,000 each and a certificate citing the awardee’s accomplishments in research.

The citation for Prof. Ho’s award explains:

“Much of the research accomplishment of Professor Ho has been on using SDSS-III data to measure cosmic distance scales and the growth of structure in the universe in order to get at the expansion history of the universe. She has been a leader in extracting signals of the Baryon Acoustic Oscillations, which are the tiny ripples in the density of galaxies that are an imprint left over from the quantum fluctuations in density soon after the Big Bang. She utilized these signals as a standard ruler to measure the distance scale of the universe in various epochs, and was able to achieve the most accurate measurements of cosmic distances yet with an accuracy of 1%. Her current research focuses on developing the understanding of dark energy via large-scale spectroscopy, investigating the initial conditions and contents of the universe large-scale photometry, and applying machine learning to studying non-linear cosmological problems.

Dr. Karen Masters, senior lecturer at the University of Portsmouth’s Institute of Cosmology and Gravitation and Director of Public Education and Outreach for SDSS-IV, has won the Women of the Future Science award. The Women of the Future Awards acknowledge successful young women in Britain and are handed out in fields ranging from business to arts and culture to science and technology. Karen (as we like to call her) received the award for her work
on understanding how galaxies form and evolve over the history of the universe. Karen uses a diverse set of tools, including the contributions of large number of citizen scientists looking at SDSS images of galaxies at the Galaxy Zoo (www.galaxyzoo.org) and the new data coming from the MaNGA survey of SDSS-IV (http://www.sdss.org/sdss-surveys/manga/). Karen is also one of the BBC’s “100 Women of 2014″, invited to share her thoughts and experiences as part of the BBC’s pledge to represent women better in their news reporting.

Dr. Masters accepting her award from the Rt Hon John Bercow MP, Speaker of the House of Commons, and Trui Hebbelink from Shell.